Method for the electrochemical synthesis of ammonia and device for carrying out said method

Abstract

A method for the electrochemical synthesis of ammonia and an apparatus for carrying out the method. The electrochemical synthesis of ammonia (NH3) involves contacting a nitrogen-containing gas with the cathode of an electrochemical cell comprising a cathode, an anode, an alkaline aqueous electrolyte, and preferably an anion exchange membrane. The cathode is a porous gas diffusion electrode comprising an active layer comprising a material capable of catalyzing the electrochemical reduction of nitrogen and in contact with the electrolyte, a layer of conductive material supporting the active layer, and a porous polymer membrane on the side of the active layer that is not in contact with the electrolyte and is in direct contact with the nitrogen-containing gas. A potential on the electrochemical cell is applied to electrochemically synthesize ammonia from nitrogen and water. The method can be used to synthesize ammonia without using molecular hydrogen.

Description

[Technical field]

[0001] The present invention relates to a method for the electrochemical synthesis of ammonia from nitrogen and water, as well as a device and a gas diffusion electrode for use in this method. [Background technology]

[0002] [Considerations on background information] Ammonia (NH3) is a chemical widely used in industry and as an agricultural fertilizer, but can also be utilized as a renewable energy storage intermediate. Currently, the production of ammonia is achieved by the energetically demanding Haber-Bosch process, which is associated with low efficiency. The century-old Haber-Bosch process for ammonia synthesis requires harsh operating conditions including high temperatures (400-500 °C) and high pressures (150-300 atm) using a heterogeneous iron-based catalyst. Global ammonia production in 2019 was 235 million tonnes, which accounts for 1-2% of the world's energy supply and demand and gives rise to about 1% of the world's energy-related CO2 emissions. An emerging alternative to the Haber-Bosch process is the electrochemical synthesis of ammonia by the nitrogen reduction reaction (NRR). In electrochemical ammonia synthesis, ammonia is formed by applying an electric potential over an electrochemical cell using a catalyst. See, for example, US Patent Application Publication No. 2016 / 0083853 A1. The electrocatalytic reduction method is considered to be an environmentally friendly approach for the production of NH3, and indeed, it can be carried out under mild conditions such as room temperature and atmospheric pressure, and can also be powered by renewable energy.

[0003] Ammonia consists of 17.6 wt% hydrogen, which makes it an indirect hydrogen storage compound. The energy density of ammonia is 4.32 kWh / L, which is comparable to that of methanol (CH3OH) and approximately twice that of liquid hydrogen. The energy required to liquefy hydrogen is greater than that of ammonia, since ammonia liquefies at -33.4°C at atmospheric pressure, whereas hydrogen must be cooled to below -253°C to liquefy. Another drawback of using hydrogen as an energy carrier is that hydrogen is difficult to transport and store without dissipating, thereby rendering it unusable for its intended end use. Furthermore, unlike hydrogen, ammonia is typically not explosive. Considering the above factors, ammonia is believed to be a preferred energy storage intermediate to hydrogen.

[0004] Several research efforts have been made to develop electrochemical ammonia synthesis based on various electrolytes. Ranging from solid oxides to molten salts, most of these efforts have focused on medium- to high-temperature systems. Although the kinetics of ammonia synthesis are better at higher temperatures, the thermodynamic efficiency at higher temperatures is lower. Parasitic energy losses in the balance of the plant to maintain the desired operating temperature represent an additional problem. Moreover, due to the high temperatures involved, the electrochemical reactor must be constructed of relatively expensive materials. Moreover, the entire system appears to exhibit poor stability in long-term operation.

[0005] Ammonia synthesis or nitrogen fixation by electrochemical means typically involves feeding an electrochemical cell with a nitrogen source, preferably pure N from purified air, and protons (H + ) is provided. The presence of water in the system, mainly from the supplied electrolyte, can result in side reactions such as water electrolysis, which can compete with both the anodic and cathodic reactions. Anodic H2 or OH - is oxidized using a suitable catalyst that can help direct the reaction to the desired product.

[0006] In view of the above, it would be advantageous to have available a system and method for the electrochemical synthesis of ammonia that can synthesize ammonia directly from water and nitrogen by using a low temperature electrolyte system and further using water as a hydrogen source via in situ water splitting during the electrochemical synthesis of ammonia. Summary of the Invention

[0007] The present invention provides a method for the electrochemical synthesis of ammonia. The method includes contacting a nitrogen-containing gas with the cathode of an electrochemical cell that includes a cathode, an anode, and an alkaline aqueous electrolyte. The cathode is a porous gas diffusion electrode that includes (i) an active layer that includes a material capable of catalyzing the electrochemical reduction of nitrogen and that is in direct contact with the electrolyte, (ii) a layer of a conductive material that supports the active layer, and (iii) a porous polymer film on a side of the active layer that is not in direct contact with the electrolyte and is in direct contact with the nitrogen-containing gas. The anode is made of a conductive material that is inert to the electrolyte. The method further includes applying a potential on the electrochemical cell to effect the electrochemical synthesis of ammonia from nitrogen and water.

[0008] In one aspect of this method, the method may be carried out at a temperature of about 20° C. to about 200° C. and / or at a pressure of about atmospheric pressure to about 10 atm, for example, at ambient (room) temperature and atmospheric pressure.

[0009] In another embodiment, the process may be carried out continuously or batchwise.

[0010] In yet another aspect of the method, the nitrogen-containing gas may be substantially pure nitrogen.

[0011] In yet another aspect of the method, a stream of nitrogen-containing gas may be contacted with the porous polymer film of the gas diffusion electrode.

[0012] In another embodiment, no molecular hydrogen (H2) is used during the electrochemical synthesis of ammonia (i.e., no molecular hydrogen that is generated in situ during the electrochemical reaction is used).

[0013] In another embodiment, the alkaline aqueous electrolyte may include a hydroxide of an alkali metal, such as Na or K, and / or a hydroxide of an alkaline earth metal, such as Mg or Ca.

[0014] In another embodiment of this method, potentiostatic or galvanostatic may be used.

[0015] The present invention also provides an apparatus for carrying out the above-mentioned method (including one or more of its various aspects). The apparatus includes an electrochemical cell including a cathode, an anode, and an alkaline aqueous electrolyte. The cathode is a porous gas diffusion electrode including (i) an active layer including a material capable of catalyzing the electrochemical reduction of nitrogen (N2) and in direct contact with the electrolyte, (ii) a layer of conductive material supporting the active layer, and (iii) a porous polymer film on the side of the active layer that is not in direct contact with the electrolyte. The anode is made of a conductive material that is inert to the electrolyte.

[0016] In one embodiment of the device, the active layer may further comprise a binder material, such as, for example, a hydrophobic polymer.

[0017] In another embodiment of the apparatus, the material capable of catalyzing the electrochemical reduction of nitrogen may include one or more of Pd, Pt, Au, Ir, Ru, Rh, Ni, Co, Mo, Cr, Ti, Zr, Hf, V, Nb, Ta, Fe, and Mn.

[0018] In yet another embodiment of the device, the layer of conductive material of the cathode may comprise a metal mesh, such as a nickel mesh, for example, and / or may be embedded in the active layer.

[0019] In another embodiment of the device, the porous polymer film may comprise or consist of a hydrophobic polymer, such as a fluorinated polymer.

[0020] In another embodiment of the device, the anode may be present in the form of a metal mesh, such as, for example, a nickel mesh.

[0021] In yet another aspect, the apparatus may further comprise a containment vessel directly adjacent to the cathode side of the electrochemical cell and including an inlet for the gas, through which a flow of a nitrogen-containing gas can be passed in contact with the polymer film of the cathode.

[0022] In another aspect of the device, the electrochemical cell further comprises a separator, such as, for example, an anion exchange membrane, separating the cathode side of the electrolyte from the anode side of the electrolyte.

[0023] In another embodiment, the apparatus may further comprise a potentiostat and / or a galvanostat.

[0024] The invention also provides a porous gas diffusion electrode suitable for use in the above-described device (including any one or more of its various embodiments), the electrode comprising: (i) an active layer comprising a material capable of catalyzing the electrochemical reduction of nitrogen (N2) in the presence of an alkaline aqueous electrolyte, (ii) a layer of conductive material supporting the active layer, and (iii) a porous polymer film on a side of the active layer opposite the side of the active layer in direct contact with the electrolyte.

[0025] In one embodiment of this electrode, the active layer may further comprise a binder, such as a hydrophobic polymer (eg, a fluorinated polymer).

[0026] In another embodiment of this electrode, the polymer film may comprise or consist of a hydrophobic polymer (e.g., a fluorinated polymer such as tetrafluoroethylene), which may be the same polymer used as the binder in the active layer.

[0027] In yet another embodiment of this electrode, the electrode may have an average pore size of about 7 nm to about 9 nm, and / or the polymer film may have a thickness of about 0.75 mm or less, and / or the electrode may have a total thickness of about 0.85 mm or less. [Brief description of the drawings]

[0028] The invention is further explained in the following detailed description, by way of non-limiting examples of exemplary embodiments of the invention, with reference to the accompanying drawings, in which: [Figure 1] The electrolytic reactor used in the experiments described below is shown diagrammatically. [Diagram 2] 1 shows a schematic diagram of a gas diffusion electrode for use in a device of the invention; [Diagram 3] 1 is a graph of current versus voltage obtained by cyclic voltammetry using a gas diffusion electrode of the present invention containing a Pt-Pd nitrogen reduction catalyst. [Figure 4] 1 is a graph of ammonia concentration versus sampling time obtained in the experiments described below. [Diagram 5] 1 is a graph of ammonia concentration versus sampling time obtained in the control experiment described below. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The details shown in this specification are presented as examples only for the purpose of illustrative discussion of embodiments of the present invention, and as a cause for providing the most useful and easily understood explanation of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show the structural details of the present invention in more detail than is necessary for a fundamental understanding of the present invention, and the description is made in conjunction with figures that will make clear to those skilled in the art how some forms of the present invention can be embodied in practice.

[0030] As used herein, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise. For example, a reference to a "gas" also means that a mixture of two or more gases may be present unless specifically excluded.

[0031] Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and the like used in the specification and appended claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and appended claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding practices.

[0032] Additionally, disclosure of a numerical range within this specification is considered to be a disclosure of all numerical values ​​and ranges within that range. For example, if a range is 1 to 50, it is considered to include, for example, 1, 7, 34, 46.1, 23.7, or any other value or range within that range.

[0033] As described above, the method of the present invention includes contacting a nitrogen-containing gas (e.g., pure nitrogen, but also air with CO2 and other gases removed that may contaminate the nitrogen reduction catalyst in the cathode) with the cathode of an electrochemical cell that includes a cathode, an anode, and an alkaline water electrolyte. The cathode is a porous gas diffusion electrode that includes: (i) an active layer that includes a material capable of catalyzing the electrochemical reduction of nitrogen (sometimes referred to herein simply as a "reduction catalyst" or "catalytic material") and that is in direct contact with the electrolyte; (ii) a layer of conductive material that supports the active layer (is a substrate for the active layer); and (iii) a porous polymer film on the side of the active layer that is not in direct contact with the electrolyte and is in direct contact with the nitrogen-containing gas. The anode is made of a conductive material that is inert to the electrolyte (e.g., the same conductive material used as conductive material (ii)). The method further includes applying a potential on the electrochemical cell to effect the electrochemical synthesis of ammonia from nitrogen and water. In a preferred embodiment, the method is carried out without the use of molecular hydrogen (other than any molecular hydrogen that may be formed in situ during the electrochemical reaction). The ability to produce ammonia without the use of molecular hydrogen is a major (and unexpected) advantage of this method. It is further preferred to use an anion exchange membrane to separate the cathode side of the electrolyte from the anode side of the electrolyte.

[0034] The method may be carried out at about 20° C. to about 200° C., and / or at about atmospheric pressure to about 10 atm. For example, the method may be (and preferably is) carried out at atmospheric pressure and ambient (room) temperature (e.g., about 20° C. to about 30° C.).

[0035] The process can further be carried out continuously or batchwise, with continuous or semi-continuous operation being preferred.

[0036] Although the nitrogen-containing gas used in the method may contain other gases in addition to nitrogen (e.g., oxygen, helium, argon, and mixtures thereof), it is typically preferred that the nitrogen-containing gas is substantially pure nitrogen, e.g., nitrogen having a purity of at least 95%, at least 98%, at least 99%, at least 99.5%, or at least 99.9% (all by volume).

[0037] The alkaline aqueous electrolyte of the electrochemical cell may be a liquid and / or gel electrolyte and will often comprise a hydroxide of an alkali metal such as Na and / or K (particularly KOH) and / or an alkaline earth metal such as Mg and / or Ca. The hydroxide concentration may range, for example, from about 0.5 M to about 9 M, e.g., from about 1 M to about 5 M, and the pH of the electrolyte will often be at least about 8, e.g., at least about 9, at least about 10, or at least about 11.

[0038] An apparatus for carrying out the method of the invention includes an electrochemical cell comprising a cathode, an anode and an alkaline aqueous electrolyte. The cathode is a porous gas diffusion electrode comprising (i) an active layer comprising a material capable of catalyzing the electrochemical reduction of nitrogen and in direct contact with the electrolyte, (ii) a layer of conductive material supporting (and serving as a substrate for) the active layer, and (iii) a porous polymer film on the side of the active layer that is not in direct contact with the electrolyte. The anode is made of a conductive material that is inert to the electrolyte.

[0039] The active layer (i) may (and preferably does) further comprise a binder material, such as a hydrophobic polymer. In general, the active layer of a gas diffusion electrode is designed to optimize the contact between the reactant gas, electrolyte, and catalyst at the so-called three-phase boundary (solid-liquid-gas in the case of liquid electrolytes) to maximize the reaction rate. The catalyst is incorporated into the active layer structure to increase the rate of the desired reaction. The catalyst is often a precious metal or its alloy in the form of a very high surface area, dispersed and supported on a high surface area conductive porous carbon black or graphite. The catalytic components may also include non-precious metals, such as one or more transition metals. In addition to the catalytic material, the active layer often also contains non-catalytic components, usually a polymeric material that acts as a binder to hold the layers together and may also have the additional function of adjusting the hydrophobic / hydrophilic balance of the final structure. The hydrophobic binder, commercially known as Teflon, often polytetrafluoroethylene (PTFE), is used mainly in two forms: as a dry powder or as a suspension. One of the best methods is the preparation of a material called Teflonized carbon black. This is done by mixing carbon black with a PTFE suspension. The result is a highly hydrophobic material with high gas diffusion rates. This material is described, for example, in U.S. Patent Nos. 3,537,906 and 4,031,033, the entire disclosures of which are incorporated herein by reference.

[0040] The polymer binder for use in the active layer (i) may be hydrophobic and may be selected from hydrophobic polymers such as fluorinated polymers (e.g., PTFE, polyhexafluoropropylene, polychlorofluoroethylene, polyvinylidene fluoride, and fluorinated ethylene-propylene copolymers), polyvinyl chloride, polyethylene, polypropylene, ethylene-propylene copolymers, polyisobutene, and combinations of two or more thereof. A preferred polymer binder for use in the active layer is PTFE. Depending on the hydrophobicity of the reduction catalyst used, the polymer binder for the active layer may be a mixture of one or more hydrophobic polymers and one or more hydrophilic polymers to adjust the hydrophilic / hydrophobic balance of the active layer to an appropriate value (to provide a sufficiently stable three-phase boundary). Non-limiting examples of suitable hydrophilic polymers for this purpose include polysulfones, perfluorosulfonate ionomers, and epoxy resins.

[0041] Binders for the active layer (i) may be selected for use in combination to ensure the appropriate hydrophobic / hydrophilic balance of the electrode, providing optimal ion conduction pathways in the electrode. A further advantage of this method is that the incorporation of polymeric materials into the structure can be carefully controlled. This provides the ability to tailor the hydrophobic / hydrophilic nature of the matrix to impart improved performance characteristics.

[0042] Materials capable of catalyzing the electrochemical reduction of nitrogen may include, for example, one or more of Pd, Pt, Au, Ir, Ru, Rh, Ni, Co, Mo, Cr, Ti, Zr, Hf, V, Nb, Ta, Fe, and Mn, either by themselves or in the form of alloys and compounds (e.g., oxides) thereof. A non-limiting list of suitable reduction catalysts includes one or more of Pd, Ni, Au, Pd-Pt, Pd-Ni, Au-Ni, Au-Pd, Au-Ni-Pd, Au-Ni-Pd, MoS-FeMoS, NiO-Cr2O3, CoO-Cr2O3, NiO-MoO3, CoO-MoO3, and CoO-Fe2O3.

[0043] The layer of conductive material (ii) (conductive substrate) must be able to withstand the alkaline conditions provided by the electrolyte and may be selected, for example, from conductive support structures known for this purpose. The material may be selected, for example, but not limited to, conductive mesh, grid, metal foam, expanded metal, and any combination thereof. A preferred conductive substrate for use in the gas diffusion electrode of the present invention is a conductive metal mesh (e.g., nickel mesh). The conductive substrate may be made of any conductive material, and is preferably made of a metallic material, such as a pure metal or a metal alloy. Other suitable conductive materials (ii) include, for example, carbon fiber, carbon paper, glassy carbon, carbon nanofibers, and carbon nanotubes. This conductive material is often embedded in the active layer.

[0044] The porous polymer film (iii) of the gas diffusion electrode of the present invention may comprise or consist of a hydrophobic polymer, which may be selected from, for example, fluorinated polymers (e.g., PTFE, polyhexafluoropropylene, polychlorofluoroethylene, polyvinylidene fluoride, and fluorinated ethylene-propylene copolymers), polyvinyl chloride, polyethylene, polypropylene, ethylene-propylene copolymers, polyisobutene, and combinations of two or more thereof. A preferred polymer binder for use in the polymer film (iii) is PTFE.

[0045] It is advantageous to use the same hydrophobic polymer for the production of the porous polymer film (iii) and the active layer (i) of the gas diffusion electrode of the invention, which ensures particularly good adhesion between the porous polymer film (iii) and the active layer (i).

[0046] The anode of the electrochemical cell may be the same as or similar to the layer of conductive material (ii) used in the cathode of the electrochemical cell. Examples of these are the same as those described above for material (ii). For example, the anode may be in the form of a metal mesh, such as a nickel mesh.

[0047] The apparatus of the present invention may further comprise a containment vessel directly adjacent the cathode side of the electrochemical cell and equipped with an inlet for the gas, through which a flow of nitrogen-containing gas can be passed in contact with the polymer film of the cathode.

[0048] The electrochemical cell of the device of the invention will typically include a separator, such as an anion exchange membrane or thin polymer film, that allows the passage of ions and separates the cathode side of the electrolyte from the anode side of the electrolyte. Anion exchange membranes are the preferred separators for use in the device.

[0049] A porous gas diffusion electrode suitable for use in the device of the present invention comprises (i) an active layer comprising a material capable of catalyzing the electrochemical reduction of nitrogen (N2) in the presence of an alkaline aqueous electrolyte, (ii) a layer of conductive material supporting (and serving as a substrate for) the active layer, and (iii) a porous polymer film on the side of the active layer opposite the side of the active layer that is in direct contact with the electrolyte. Details of the electrode are given above.

[0050] In an exemplary embodiment, the gas diffusion electrode of the present invention is usually in the form of a sheet and may have an average pore size of about 7 nm to about 9 nm. The polymer film (iii) may have a thickness of, for example, about 0.75 mm or less, for example, about 0.3 mm to about 0.7 mm. The total thickness of the gas diffusion electrode of the present invention is preferably about 0.85 mm or less, and may be about 0.5 mm.

[0051] [Experimental Section] The electrochemical reactor used in the tests described below was a 10 cm 2 and is shown diagrammatically in FIG. 1, with the following abbreviations used: WE = working electrode (cathode) CE = counter electrode (anode) RE = reference electrode WG = Gas used AEM = Anion Exchange Membrane (Separator) The general structure of the cathode used is shown in FIG. 2, where 1 is the PTFE film, 2 is the active layer and 3 is a nickel mesh (partially) embedded in the active layer 2.

[0052] Cathode preparation: 25 g of catalyst (Pd-Pt deposited on carbon) was mixed with PTFE in a weight ratio of 80 / 20. Mixing was performed at room temperature for about 15-30 minutes using a blender. The resulting mixture of catalyst and PTFE was placed in a rolling machine to obtain a catalyst layer thin film. The resulting catalyst layer was embedded in a nickel mesh. Next, a porous PTFE membrane was applied to the surface of the catalyst layer opposite to the nickel mesh side to obtain an electrode.

[0053] The cathode electrolyte chamber was separated from the anode electrolyte chamber by a separator, which was purchased from Fuma-Tech and consisted of a long-chain perfluorosulfonic acid polymer (Fumion® F).

[0054] Two different electrolytic cells with two electrolyte chambers were used: one cell had a total volume of 660 ml (330 ml per chamber) and the other had a total volume of 360 ml.

[0055] The method was carried out at room temperature and atmospheric pressure.

[0056] A mercury / mercury oxide-Hg / HgO (hereafter "MMO") reference electrode was filled with 6.6 M KOH and connected to the cell by a salt bridge. Pure nitrogen gas (99.99%) was flowed through the electrolyte solution and the cathode gas chamber. Nitrogen or argon was flowed through the anode electrolyte chamber to remove dissolved gases in the solution.

[0057] The cathode electrolyte and gas chamber outlets were connected to a trap containing a dilute solution of sulfuric acid (H2SO4) to hold the synthetic ammonia as the ammonium salt.

[0058] An electrical device (potentiostat / galvanostat) was connected to the reactor to apply a constant potential or a constant current, respectively, to the system.

[0059] The operating procedure for the ammonia synthesis cell was as follows.

[0060] Once the cell was assembled and all electrodes were in place, the cell was filled with fresh electrolyte (aqueous KOH, 1M, 5.4% w / w).

[0061] Fresh sulfuric acid solution was placed in the trap and connected to the cathode gas outlet.

[0062] To remove dissolved oxygen, N2 gas was bubbled into the cathode electrolyte chamber, as was the anode electrolyte chamber with argon or N2.

[0063] The electrodes were connected to a potentiostat / galvanostat to measure the OCV (open circuit voltage) of the system. Chronoamperometry was applied at various potentials between -0.9 V and -1.1 V vs. MMO for a set period of time while the gas continued to flow into the chamber and through the acid trap.

[0064] The solution in the acid trap was sampled periodically and tested for the presence of ammonia. Current and electrode potentials (cathode and anode) were recorded and observed throughout the operation of the cell.

[0065] The concentration of ammonia produced was determined by colorimetry using a UV VIS photometer. Two identification and quantification methods were used, one utilizing Nessler's reagent-potassium tetraiodomercurate(II) and the other based on the Berthelot reaction with salicylic acid as an indophenol derivative. The first step in the quantification of ammonia was to establish calibration curves for both Nessler's reagent and the Berthelot reaction using known concentrations of ammonium chloride as the precursor of ammonia.

[0066] Ammonium chloride solutions of known concentrations were prepared from a 1000 ppm stock solution. NH4Cl chloride samples ranging from 1 ppm to 6 ppm were prepared using Nessler's reagent and the Berthelot reaction, and the resulting absorbance measurements were plotted to form a calibration curve.

[0067] The constant potential applied to a particular electrode was determined by testing the electrode with cyclic voltammetry to determine when water reduction begins, a process required for ammonia synthesis. The results are shown in Figure 3.

[0068] [result] Under the above conditions, a concentration of 2.8 mg / L of ammonia was detected after 14 hours of operation. This concentration was observed in the working electrode (cathode) trap. No ammonia was detected in the counter electrode trap, which was used as a control. Control experiments such as replacing the nitrogen gas flow with argon (see FIG. 5) or removing the applied potential all confirmed that the observed ammonia was a product of electrochemical synthesis.

[0069] FIG. 4 shows the concentration of ammonia as a function of sampling time at a constant potential of −1.1 V versus the MMO working electrode trap. The measured total ammonia concentration is the sum of the ammonia detected in the cathodic acid trap and in the cathodic electrolyte solution. The upper curve in FIG. 4 shows the combined ammonia concentration, while the lower curve shows the ammonia concentration in the working electrode trap, as partially described in the table below. FIG. 5 shows the performance of a control test without N2. [Table 1]

[0070] The amount of ammonia obtained was calculated from the corresponding ppm concentrations measured in the acid trap and electrolyte samples, taking into account the volume of the total solution in the acid trap and electrolyte. The total amount of ammonia obtained was 0.35 mg ammonia cm of the electrode surface. 2 per hour and cm of electrode surface. 2The concentration of ammonia was 2.5 μg per sample.

Claims

1. Ammonia (NH ３ 1. A method for the electrochemical synthesis of nitric acid (NiO), comprising contacting a nitrogen-containing gas with a cathode of an electrochemical cell comprising an anode and an alkaline aqueous electrolyte, wherein the cathode contains: (i) nitrogen (N ２ (ii) an active layer comprising a material in direct contact with the electrolyte capable of catalyzing the electrochemical reduction of nitrogen-containing gas; (iii) a porous gas diffusion electrode comprising: (i) an active layer comprising a material in direct contact with the electrolyte capable of catalyzing the electrochemical reduction of nitrogen; (ii) a layer of conductive material supporting the active layer; and (iii) a porous polymer film on a side of the active layer that is not in direct contact with the electrolyte but is in direct contact with the nitrogen-containing gas; wherein the anode is made of a conductive material that is inert to the electrolyte; and the method comprises applying an electric potential across the electrochemical cell to effect the electrochemical synthesis of ammonia from nitrogen and water.

2. 10. The method of claim 1, wherein the method is carried out at a temperature of from about 20° C. to about 200° C. and / or at a pressure of from about atmospheric pressure to about 10 atm.

3. The method of claim 1 , wherein the method is performed continuously.

4. The method of claim 1 , wherein the method is carried out batchwise.

5. The method of claim 1 , wherein the nitrogen-containing gas is substantially pure nitrogen.

6. The method of claim 1 , wherein the nitrogen-containing gas stream is contacted with the porous polymer film of the gas diffusion electrode.

7. Molecular hydrogen (H ２ 2. The method of claim 1, wherein no .

8. The method of claim 1 , wherein the aqueous electrolyte comprises a hydroxide of an alkali metal and / or an alkaline earth metal.

9. The method of claim 1 , wherein a constant potential is used.

10. The method of claim 1 wherein a constant current is used.

11. The present invention includes an electrochemical cell comprising a cathode, an anode, and an alkaline aqueous electrolyte, the cathode comprising: (i) nitrogen (N ２ 11. An apparatus for carrying out the method of claim 1 , wherein the anode is made of a conductive material that is inert to the electrolyte, and the anode is a porous gas diffusion electrode comprising: (i) an active layer comprising a material in direct contact with the electrolyte that is capable of catalyzing the electrochemical reduction of HCl; (ii) a layer of conductive material supporting the active layer; and (iii) a porous polymer film on a side of the active layer that is not in direct contact with the electrolyte, the anode being made of a conductive material that is inert to the electrolyte.

12. The device of claim 11 , wherein the active layer further comprises a binder material.

13. The apparatus of claim 12 , wherein the binder material comprises a hydrophobic polymer.

14. 12. The apparatus of claim 11, wherein the material capable of catalyzing the electrochemical reduction of nitrogen comprises one or more of Pd, Pt, Au, Ir, Ru, Rh, Ni, Co, Mo, Cr, Ti, Zr, Hf, V, Nb, Ta, and Mn.

15. The apparatus of claim 11 , wherein the layer of conductive material of the cathode comprises a metal mesh.

16. The device of claim 11 , wherein the layer of conductive material of the cathode is embedded in the active layer.

17. The device of claim 11 , wherein the porous polymer film comprises a fluorinated polymer.

18. The device of claim 11 , wherein the anode is in the form of a metal mesh.

19. 12. The apparatus of claim 11, further comprising a containment vessel adjacent the cathode side of the electrochemical cell and including an inlet and an outlet for the gas, the containment vessel allowing a flow of nitrogen-containing gas to pass through in contact with the polymer film of the cathode.

20. The device of claim 11 , wherein the electrochemical cell further comprises a separator.

21. 21. The device of claim 20, wherein the separator comprises an anion exchange membrane.

22. The apparatus of claim 11 further comprising a potentiostat and / or a galvanostat.

23. (i) Nitrogen (N ２ 12. A porous gas diffusion electrode suitable for use in the device of claim 11, comprising: (i) an active layer comprising a material capable of catalyzing the electrochemical reduction of ZnO; (ii) a layer of conductive material supporting the active layer; and (iii) a porous polymer film on a side of the active layer opposite the side of the active layer in contact with the electrolyte.

24. 24. The electrode of claim 23, wherein the active layer further comprises a binder.

25. 25. The electrode of claim 24, wherein the binder comprises a hydrophobic polymer.

26. 26. The electrode of claim 25, wherein the hydrophobic polymer comprises a fluorinated polymer.

27. 24. The electrode of claim 23, wherein the porous polymer film comprises a fluorinated polymer.

28. 27. The electrode of claim 26, wherein the fluorinated polymer of the active layer and the fluorinated polymer of the polymer film are the same.

29. 27. The electrode of claim 26, wherein the fluorinated polymer is polytetrafluoroethylene (PTFE).

30. 24. The electrode of claim 23, wherein the electrode has an average pore size of about 7 nm to about 9 nm.

31. 24. The electrode of claim 23, wherein the polymer film has a thickness of about 0.75 mm or less.

32. 24. The electrode of claim 23, wherein the electrode has a total thickness of about 0.85 mm or less.

33. A method for the electrochemical synthesis of ammonia (NH ３ ), which is carried out without using molecular hydrogen (H ２ ), comprising contacting a nitrogen-containing gas with the cathode of an electrochemical cell comprising a cathode, an anode, an alkaline aqueous electrolyte, and an anion exchange membrane for separating the cathode side of the electrolyte from the anode side of the electrolyte, wherein the cathode comprises (i) an active layer comprising a material capable of catalyzing the electrochemical reduction of nitrogen and in direct contact with the electrolyte, (ii) a layer of a conductive material supporting the active layer, and (iii) a porous polymer film on the side of the active layer that is not in direct contact with the electrolyte and is in direct contact with the nitrogen-containing gas, and the anode is made of a conductive material that is inert to the electrolyte, and the method comprises applying a potential across the electrochemical cell to effect the electrochemical synthesis of ammonia from nitrogen and water.

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